1. Field of the Invention
The invention relates to valve seat inserts, to water-cooled internal combustion engines including such valve seats and to methods of manufacturing such engines.
2. Review of the Prior Art
A known valve seat insert and its arrangement in a water-cooled internal combustion engine is shown in FIG. 1 of the drawings which is a schematic cross-sectional view through an exhaust port of an internal combustion engine. The water-cooled internal combustion engine comprises at least one cylinder 10 and a valve controlled exhaust port 11 communicating with the cylinder 10. The exhaust port 11 is formed by a wall of a casting 12 which is commonly of cast iron or aluminium alloy. An exterior surface 13 of the casting defines a portion of the combustion chamber 10 and also forms the walls of the exhaust port 11. An interior surface 14 of the casting 12 defines a section of the water-cooling system of the engine to allow water 15 to circulate round the cylinder 10 and the exhaust port 11.
The casting is formed with an annular rabbet 16 around the end of the exhaust port 11 and an annular valve seat insert 17 is received in the rabbet. The valve seat insert 17 is made from a material better able than the cast iron or the aluminium alloy of the casting 12 to withstand the temperatures and impacts encountered in the hot exhaust gases as they pass into the port. The valve seat insert 17 has the usual bevelled annular seating surface 18 for engagement with a head of the valve.
Due to the high temperatures encountered in the combustion chamber 10 and in the exhaust gases passing into the exhaust port 11, the valve head and the valve seat reach elevated temperatures. For example, the surface 19 of the valve seat insert 17 which faces the cylinder 10 can reach a temperature of about 400.degree. C. and the valve-engaging surface 18 may reach a temperature of about 480.degree. C. Correspondingly higher temperatures are attained in the valve itself, with the edge of the valve head reaching, for example, about 550.degree. C. and the centre of the valve head reaching as much as about 800.degree. C. This has a number of disadvantages. It requires the valve to be made of expensive alloys such as cobalt-including alloys, particularly where the engine is a high specific output gasoline or turbo-charged diesel engine, where especially high temperatures are encountered. It can also cause valve burn-out and may eventually cause the valve seat 17 to drop out of the rabbet 16. In addition, the valve head and the valve seat insert may act as a `hot spot`, so inducing detonation and pre-ignition in a gasoline engine.